High frequency power amplifiers are crucial elements in a variety of radio frequency circuit applications and are challenging analog circuits to design. In traditional monolithic microwave integrated circuit (MMIC) implementations of power amplifiers, the outputs of many small power transistors are combined using corporate power combining techniques. These techniques are lossy, narrow band, and waste die area on an MMIC. Power combining using spatial techniques is an emerging technological approach that seeks to overcome these limitations. One promising approach is the use of MMIC's attached to tapered slot antenna cards stacked in waveguide. See U.S. Pat. No. 5,736,908. Significant amounts of high frequency power can be generated using this approach, but the circuitry is unstable and the antennas are too large. Accordingly, there is a need for a stable wide band power amplifier of reasonable size that can be used to produce significant amounts of microwave and millimeter wave power.